UV aging induces colloidal-like behavior in microplastics, mediating contaminant fluxes across interfaces

Microplastics (MPs) are often regarded as pollutants vectors, yet they undergo substantial physicochemical transformations in aquatic environments. Here, we reveal how photo-oxidation (UVs) and mechanical stress transform polyethylene-based-MPs into reactive porous particles with colloidal behaviors. Using multiscale imaging, spectroscopy, and electrokinetic analyses, we show that aged MPs (100 ± 200 µm) develop dual porosity, pronounced surface oxidation (carbonyl index 2 ± 2), and increased roughness (Ra 13 ± 5 µm). Trace elements like titanium, chromium, and iron persist within the particles and accumulate at their surface. These changes modulate their behavior: in freshwater, negative surface charges (ζ ≈ -32 mV) promote dispersion and additive release, while in brackish and sea waters, charge screening triggers aggregation and sedimentation independent of biofilm growth. Our findings redefine MPs as dynamic and reactive entities, capable of transporting legacy additives and metals vertically and laterally through aquatic systems and into the atmosphere.